In recent years, a system for the steering of an automobile, which is so-called a “steer-by-wire system”, has been developed for practical use in variety of manners. Where the steer-by-wire system is used, it is generally practiced that either one of the following mechanisms is prepared in advance in order to secure the steering capability just in case a failure or defect occurs in the system. That is, prepared are either a coupling mechanism for mechanically coupling a steering wheel (handwheel) with turning wheels, separately from the steer-by-wire system, or its equivalent mechanism, which is so-called a fail-safe mechanism.
Also, the steer-by-wire system detects an input entered by a vehicle's driver, such as a torque and a steering angle, by sensors. And the steer-by-wire system combines this detected input with the information acquired from other sensors provided in the vehicle, and thereby derives a steering angle suitable for the running condition of the vehicle. Then the steer-by-wire system sends this steering command value to a steering actuator so as to actually steer the wheels (i.e., the turning wheels). The output of a motor used as an actuator for turning the wheels is determined on the assumption of a steering wheel operation (during stationary steering) at the time when the force required for the turning of the wheels is the maximum while the vehicle is being stopped. For this reason, a high-output and large-sized motor is required then.
In the light of this, a vehicle steering apparatus is proposed (see Patent Document 1 listed in the following Related Art Documents) in order that a small-sized, low-priced and low-output motor can be used. That is, in this vehicle steering apparatus, a clutch mechanism is coupled during stationary steering and thereby a steering mechanism and a turning mechanism are mechanically coupled together. And the drive of a turning power motor and a reaction-force motor is controlled so as to turn the wheels. By employing this vehicle steering apparatus, it seems that the output of the turning power motor during stationary steering can be suppressed and the size of the turning power motor can be made small.